The desulfurization of hydrogen sulfide-containing gas mixtures produced, for example, in chemical processing, gas purification or combustion installations is of great significance from an economical viewpoint--sulfur recovery--and for ecological reasons--air quality.
Desulfurization in sulfur recovery installations mostly operate according to the Claus process. With this method, part of the hydrogen sulfide is partially oxidized in a first reaction step in a combustion chamber to sulfur dioxide: EQU 2H.sub.2 S+30.sub.2 .revreaction.2SO.sub.2 +2H.sub.2 O
In subsequent catalytic reaction steps, the produced sulfur dioxide is converted to sulfur with non-oxidized hydrogen sulfide: EQU 2H.sub.2 S+SO.sub.2 .revreaction.3S+2H.sub.2 O
In order to set the stoichiometric ratio of H.sub.2 S: SO.sub.2 =2 : 1 required for these reaction steps an effort must be made to oxidize in the first reaction step as accurately as possible one third of the hydrogen sulfide to sulfur dioxide by means of an appropriate air supply. Since the content of hydrogen sulfide in the operation gas is, however, subject to fluctuations and, in addition, other combustible gases such as hydrocarbons may be present in varying amounts which changes the oxygen demand, the determination of the oxygen demand and the corresponding regulation of the oxygen supply is of great importance in these installations.
This is of course also applied to waste gas purification installations added to a Claus installation in which for process reasons, an H.sub.2 S/SO.sub.2 ratio must be set which differs from 2. These include, for example, the "Sulfreen" process and the thermal post treatment of the Scot Process.
Many proposals were made, therefore, for regulation systems for adjusting the air/oxygen supply to the actual demand. U.S. Pat. No. 3,945,904, for example, describes a method in which the H.sub.2 S and SO.sub.2 content in the waste gas leaving the Claus installation is determined. To analyze these gases, titration instruments are used and their signals regulate the air supply. According to U.S. Pat. No. 4,100,266 an analysis is conducted of the H.sub.2 S and SO.sub.2 leaving the Claus process. Gas chromatographs are proposed here as analysis instruments and their signals are processed in a process computer and are used for the control of valves which set the desired ratio of air to hydrogen sulfide-containing gas. According to U.S. Pat. Nos. 3,985,864 and 4,021,201 an analysis of the H.sub.2 S and SO.sub.2 in the waste gas flow of a Claus installation takes place in a similar manner whereby the obtained signal affects the regulation system for the air supply accordingly.
All these methods, however, have the disadvantage that the regulation system can only intervene with a correction when the installation has already departed from its optimum operating condition.
In some of the described methods, for example, in the above cited U.S. Pat. Nos. 3,985,864 and 4,021,201 provisions are made, to be sure, to immediately correct changes in the pressure of the operation gas, for example, when the regulation system is connected to pressure and throughput meters but these inherently can only consider physical changes of the operation gases. Concentration changes in the operation gas, for example, a change in the hydrogen sulfide concentration, the hydrocarbon content or a change in the chemical composition of the hydrocarbons can only be detected in the waste gas analysis. These methods fail, therefore, with rapid composition changes in the operation gas.
A method is also known (U.S. Pat. No. 3,854,876) which avoids some of these problems by taking a sample of the operation gas and/or waste gas which is subjected to complete combustion in a furnace. After condensation of the water vapor and separation of all liquid and solid components in appropriate traps and filters, the carbon dioxide content is measured in a first analyzer and the sulfur dioxide content is measured in a second analyzer The gas mixture then goes through an oxygen analyzer designed as a monitor and a warning signal is emitted when a predetermined minimum oxygen content is not present. The two analyzers for carbon dioxide and sulfur dioxide content emit electrical signals corresponding to the measured CO.sub.2 and SO.sub.2 concentration which are processed in an analog computer and are finally used for regulating a process variable for the purpose of optimizing the sulfur recovery.
Photometers are proposed for the rapid and continuous analysis, which measure the carbon dioxide concentration by infrared absorption and the sulfur dioxide concentration by UV absorption.
But this method also has several disadvantages. The gas mixture to be analyzed must be carefully relieved of water vapor since otherwise the IR measurement of the CO.sub.2 content is subject to interference. The installation of coolers, filters and liquid traps in the line system increases the time before the analysis results are available so that the regulation is also delayed. In the water vapor condensation, moreover, the part of CO.sub.2 and SO.sub.2 dissolved in the water escapes the measurement by the photometer. This percentage, furthermore, fluctuates with the temperature which determines the solubility. The determination of the carbon dioxide content finally does not permit a safe conclusion on the type of hydrocarbon contained in the sample gas and, therefore, on the oxygen demand of the process since saturated, unsaturated and aromatic hydrocarbons have a different oxygen demand which is not proportional to the number of carbon atoms. A carbon dioxide content in the operation gas and sample gas, moreover, falsifies the result with respect to oxygen demand.
It was, therefore, the objective of the invention to provide a method for predetermining reliably and quickly the oxygen demand of a desulfurization or sulfur recovery installation from an analysis of the operation gas mixture to detect momentarily occurring fluctuations in the composition of the operation gas and to confront these fluctuations by counter measures.
The solution of this objective consists of a method for continuously monitoring and predetermining the oxygen demand in the combustion of hydrogen sulfide-containing gas mixtures for sulfur recovery and waste gas purification by continuous sample taking of the operation gas, its complete combustion in a combustion furnace (123) with at least the stoichiometric oxygen amount and analysis of two components of the burned gas mixture in continuously operating analysis instruments with their output signals used for regulating the oxygen supply to the main gas flow and is characterized in that the analytically detected components of the burned gas mixtures are oxygen and sulfur dioxide.